Molecular Shuttling in Crown Ether@ZIF-8 for CO2 Separation and Capture

Polymeric membranes for CO 2 capture often face a trade-off between permeability and selectivity. Here, crown ethers were introduced as molecular shuttles for CO 2 transport by in situ encapsulation within ZIF-8 nanocages. Three crown ethers (18-crown-6 (18C6), diaza-18-crown-6 (DAC), and dibenzo-18-crown-6 (DBC)) were incorporated into ZIF-8 during crystallization and then used as low-loading fillers in Pebax mixed-matrix membranes. Structural characterization by XRD, FTIR, SEM/EDS, and BET confirmed that ZIF-8 crystallinity was preserved, while pore volume, linker rotation, and filler dispersion were effectively tuned. These modifications narrowed transport pathways and strengthened polymer–filler interactions, improving CO 2 separation performance. At an optimal loading of 1 wt% 18C6@ZIF-8, the membrane doubled CO 2 permeability relative to neat Pebax and increased CO 2 /N 2 selectivity from 55.5 to 85.8, surpassing the 2008 Robeson upper bound. The membrane also maintained stable performance under feed pressures up to 20 bar and exceeded the 2019 Robeson upper bound. Mixed-gas testing showed a CO 2 permeability of 158.3 Barrer and CO 2 /N 2 selectivity of 66.0, outperforming neat Pebax. Transport analysis revealed that the enhancement mainly arose from improved diffusion selectivity, with a minor contribution from solubility selectivity. Among the fillers, 18C6@ZIF-8 provided the best permeability–selectivity balance, demonstrating a promising strategy for advanced CO 2 -selective mixed-matrix membranes. This approach paves the way for the development of next-generation mixed-matrix membranes (MMMs) with enhanced performance for CO 2 capture and a range of other gas-separation applications.